Fifth wheel devices include a plate which supports the forward bed plate of a trailer. A king pin is fixed to the trailer bed plate and extends into a center opening of the fifth wheel plate. Fifth wheels have included a pivotally mounted jaw to engage and hold the king pin. Usually the king pin includes a reduced diameter neck and an enlarged end and the fifth wheel jaw surrounds the neck of the king pin so as to prevent it being disengaged either vertically or horizontally. Normally the jaw closes and is secured on the king pin as the tractor and trailer are brought together. For over the road tractors there is also a fail-safe jaw unlocking and locking mechanism including a manually operated rod and latch, such as that shown in my U.S. Pat. No. 4,871,182, purposely located at the side of the fifth wheel where it can be reached only while the tractor is not in motion. However, there is an automatic unlocking system available on certain fifth wheels that are intended for non-highway use, such as in freight terminals, where operation is at low speeds and disengagement of the trailer king pin is, while undesirable, not as dangerous as at highway speeds and the ability to rapidly unlock and release a trailer is highly advantageous.
Such an automatic system has been employed on yard-captive tractors that are essentially dedicated to non-highway use; however, occasionally a yard-captive tractor has been utilized for over the highway hauling. The automatic system heretofore in use has simply substituted a powered reciprocal element, such as pneumatic cylinder, for the rod and latch and the powered element is controlled from the cab of the yard-captive tractor. Thus an operator of the tractor has been able to activate the unlocking system and permit the jaw to open to release a trailer king pin by simply moving a switch in the tractor cab. While advantageous for shifting trailers within a confined terminal, such an automatic system has not heretofore been sufficiently safe and fail proof for highway operation.